1. Field of the Invention
The present invention relates to an anti-reverse connection circuit for a power supply, and particularly to an anti-reverse connection circuit for a power supply, which prevents a reverse voltage from being applied to a load such as a control circuit for fit-up equipment activated by in-vehicle electricity, due to a reverse connection of an external power supply such as a battery to the load.
2. Description of the Related Art
A conventional power-supply anti-reverse connection circuit will be described with reference to FIG. 6. In FIG. 6, the power-supply anti-reverse connection circuit comprises an input terminal 11 connected to a positive polarity of an external power supply (BATT), an input terminal 12 connected to a negative polarity of the external power supply, load terminals 13 and 14 to which a load is connected, and a rectifier cell or device (e.g., diode) 15.
Further, the diode 15 is placed between the input terminal 11 and the load terminal 13. The anode of the diode 15 is connected to the input terminal 11, and the cathode thereof is connected to the load terminal 13. Further, the input terminal 12 and the load terminal 14 are directly connected to each other.
When the external power supply is correctly connected in such a configuration, i.e., when a positive-polarity voltage (e.g., +12V) is applied to the input terminal 11 from the external power supply and a negative-polarity voltage (e.g., 0V) is applied to the input terminal 12, the diode 15 is brought to a conducting state, so that the voltage supplied from the external power supply is applied between the load terminals 13 and 14 through the diode 15. The voltage applied at this time results in a voltage obtained by subtracting a voltage drop (which ranges from 0.6V to 0.7V) developed across the diode 15 from the voltage supplied from the external power supply.
When the external power supply is connected in the backward direction, i.e., when the negative-polarity voltage (e.g., 0V) is applied to the input terminal 11 and the positive-polarity voltage (e.g., +12V) is applied to the input terminal 12, the diode 15 is brought to a non-conducting state, so that a current flowing between the input terminal 11 and the load terminal 13 is cut off. Thus, no voltage is applied between the load terminals 13 and 14.
As described above, the conventional power-supply anti-reverse connection circuit has used rectifying action of the diode to prevent the application of a reverse voltage to a load when the external power supply is connected in the backward direction.
However, when the current used up or consumed by the load increases, power consumption of the rectifier cell or device (diode 15) increases. It is necessary to increase the absolute maximum rating for the rectifier device for the purpose of solving it. It is therefore necessary to increase the size of the rectifier cell itself. The price of the rectifier device also rises and a structure for its radiation becomes complex, thereby exerting pressure on the size of a product. Thus, the price of each product which utilizes such a power-supply anti-reverse connection circuit, also has increased.
Further, since the voltage is applied to the load through the rectifier device within the power-supply anti-reverse connection circuit, a voltage drop (which ranges from 0.6V to 0.7V) based on a PN junction of the rectifier device takes place in the circuit. Therefore, this interferes with the operation of a load circuit and the efficiency is greatly reduced.
The present invention has been made to solve the problems referred to above. An object of the present invention is to provide a power-supply anti-reverse connection circuit which prevents a reduction in the voltage applied to a load and eliminates the need for a layout with respect to radiation even if current consumption of the load increases, and which avoids an increase in price.
In order to-solve the above problems, the present invention comprises a first input terminal to be connected to a positive polarity of an external power supply, a second input terminal to be connected to a negative polarity of the external power supply, a first load terminal to which one end of a load is connected, a second load terminal to which the other end of the load is connected, a relay having a normally opened contact and an excitation coil, and a control circuit for allowing a current to flow in the excitation coil. Either one of both the first input terminal and the first load terminal, and the second input terminal and the second load terminal is connected to each other via the normally opened contact and the other thereof is directly connected to each other. The control circuit causes the current to flow in the excitation coil only when the positive polarity is connected to the first input terminal and the negative polarity is connected to the second input terminal. Thus, no heat is generated even if the current used up or consumed by the load increases or the current consumption of the load is made great, and no voltage drop is developed between the voltage supplied from the external power supply and each of the load terminals. Accordingly, the efficiency is not reduced. Further, an increase in cost is not produced either.
In the present invention as well, when the voltage supplied from the external power supply is lower than a predetermined voltage, the control circuit inhibits the flow of the current in the excitation coil. Thus, since no voltage is applied to the load when the voltage supplied from the external power supply is low, a failure in load, a malfunction thereof, the shortening of life of the external power supply, etc. can be prevented from occurring.
Further, in the present invention, the control circuit has a resistance-based voltage divider circuit connected between the first input terminal and the second input terminal, and a first switch transistor. The collector and emitter of the first switch transistor are respectively connected to the first input terminal and the second input terminal in such a manner that a current flows from the first input terminal to the second input terminal through the collector and emitter of the first switch transistor when the first switch transistor is turned on. The excitation coil is interposed in a passage through which the current flows. A voltage divided by the voltage divider circuit is applied to the base of the first switch transistor. Thus, since the current flowing in the excitation coil is turned on/off in association with the turning on/off of the switch transistor according to only the connection or disconnection of the power supply, the switching between the turning on and off of the current in the excitation coil can easily be performed.
Furthermore, in the present invention, the control circuit includes a zener diode whose cathode is connected to the first input terminal, a bias resistor connected between the anode of the zener diode and the second input terminal, and an NPN type second switch transistor of which the collector is connected to the first input terminal and the emitter is connected to the second input terminal. The base of the second switch transistor is connected to a point where the zener diode and the bias resistor are connected to each other. The excitation coil is interposed either between the collector of the second switch transistor and the first input terminal or between the emitter of the second switch transistor and the second input terminal. Thus, since the switch transistor is not turned on when the voltage supplied from the external power supply is less than or equal to a zener voltage, the setting of the voltage applied to the load can easily be set to an allowable minimum voltage.
Still further, in the present invention, the control circuit includes a zener diode whose anode is connected to the second input terminal, a bias resistor connected between the cathode of the zener diode and the first input terminal, and a PNP type third switch transistor whose collector is connected to the second input terminal and whose emitter is connected to the first input terminal. The base of the third switch transistor is connected to a point where the zener diode and the bias resistor are connected to each other. The excitation coil is interposed either between the collector of the third switch transistor and the second input terminal or between the emitter of the third switch transistor and the first input terminal. Thus, since the switch transistor is not turned on when the voltage supplied from the external power supply is less than or equal to the zener voltage even if the switch transistor is a PNP type, the setting of the voltage applied to the load to an allowable minimum voltage can easily be performed.
Still further, in the present invention, the first switch transistor is an NPN type. A capacitor for noise elimination is connected between the base of the first switch transistor and the second input terminal. An anti-reverse current diode is interposed in a passage through which a current flows from the first input terminal to the second input terminal through the collector and emitter of the first switch transistor when the first switch transistor is turned on. Thus, it is possible to prevent the switch transistor from malfunctioning due to an abnormal or improper voltage and prevent the switch transistor from destroying due to a reverse voltage.
Still further, in the present invention, the first switch transistor is a PNP type. A capacitor for noise elimination is connected between the base of the first switch transistor and the first input terminal. An anti-reverse current diode is interposed in a passage through which a current flows from the first input terminal to the second input terminal through the collector and emitter of the first switch transistor when the first switch transistor is turned on. Thus, even if the switch transistor is of the PNP type, the switch transistor can be prevented from malfunctioning due to an abnormal or improper voltage, and the switch transistor can be prevented from being destroyed due to a reverse voltage.
Still further, in the present invention, a capacitor for noise elimination is connected between the base of the second switch transistor and the second input terminal. An anti-reverse current diode is interposed in a passage through which a current flows from the first input terminal to the second input terminal through the collector and emitter of the second switch transistor when the second switch transistor is turned on. Thus, the switch transistor can be prevented from malfunctioning due to an abnormal voltage, and the switch transistor can be prevented from being destroyed due to a reverse voltage.
Still further, in the present invention, a capacitor for noise elimination is connected between the base of the third switch transistor and the first input terminal. An anti-reverse current diode is interposed in a passage through which a current flows from the first input terminal to the second input terminal through the collector and emitter of the third switch transistor when the third switch transistor is turned on. Thus, the switch transistor can be prevented from malfunctioning due to an abnormal voltage, and the switch transistor can be prevented from being destroyed due to a reverse voltage.